Back in 2007 I posted this image of a dark hole on Mars. In late 2011 they took additional images of a similar feature with the HiRISE cameras on Mars Reconnaissance Orbiter. Turns out it is in fact a “sky light” or a hole in the surface to an underground cavern. This one spans 35 meters (115 feet) across and 20 meters (65 feet) deep.
It’s origins are likely due to ancient lava flows that continued underground and eventually were drained to leave behind a cavern. How vast such a system might be is yet unknown.